Figure 2. Summary of the procedure and signal transformations.

Block 1 illustrates the relationship between the single cell currents and the macroscopic field potentials recorded in ideal DC-coupled mode. A rhythmic excitatory drive into a discrete dendritic domain establishes local sinks of current, surrounded by sources (CSD). These spatially-aligned sources and sinks produce laminar field potentials of uneven strength and polarity, whose CSD analysis renders a correct estimation of sources and sinks amplitudes and locations. Block 2 illustrates the effect of AC-coupling on recordings. Note that channels are individually filtered (zeroing) and thus each is offset by a different amount (red dotted lines). Consequently, each gamma wave is transformed into a biphasic sequence at any location. CSD analysis of the AC-coupled profile results in spurious sequences of sources and sinks at all sites. Each LFP wave (dashed ovals) returns a source/sink pair, while the lack of a true reference baseline confounds their initiation time and does not allow us to ascertain which of sources or sinks are expected in cell domains. Block 3 illustrates the rectification procedure. First the ICA decomposes the original signals into pathway-specific generators (pairs s₁t₁. s_nt_n; only one is used for simplification), each with a spatial distribution and a temporal activation (time course). Note that the curve of spatial distribution is proportional to the collection of offsets introduced by AC-coupling (zeroing in Block 2), whilst the time evolution is unique. The experimental determination of the active synaptic sites and the excitatory/inhibitory chemical nature allows offsetting the time envelope as required in order to achieve homogeneous polarity (red vertical arrow). The subsequent reconstruction using the rectified time envelope (green curved arrows) regenerates LFPs with the correct baseline at each recording site. Consequently, the application of CSD analysis generates spatiotemporal maps of sources and sinks in which individual waves have the correct amplitude and duration.